In many cases it is of interest to be able to measure the density of materials without damaging or changing it, and to be able to determine the presence and the amounts of materials having different densities. For instance, within the forest industry the possibility of distinguishing between different qualities is becoming increasingly important. Already at the felling stage it is important to know the density of the logs, on the one hand in order to be able to directly detect and to reject parts that are decayed or otherwise damaged and consequently without value, and on the other to be able to calculate the price of the timber (which at least in Sweden nowadays is set on the basis of density and not on volume). Also in sawmills density measuring is of interest. Improved knowledge of logs makes log classification easier, allowing sorting-out of damaged logs or logs exhibiting too many knots or being resinous.
Also in other branches measurements of this type may be of great importance. For instance, they make it easy to determine the quality of oil in order to estimate the amount of water, metal impurities and other components contained therein, which is important in order to allow engine oil changes to be performed at sufficiently frequent intervals and to prevent wear and breakdown. In addition, the method may be used in the handling of waste to be sorted according to type, to determine the quality of building elements, and so on.
For the purpose of measuring density, it has been known for a long time to allow electromagnetic radiation to penetrate an object to be measured and thereafter to measure the intensity and to calculate the amount of the original intensity that has been absorbed. Examples of such methods and devices are found in SE 466 365, DE 28 46 702, U.S. Pat. No. 3,136,892 and U.S. Pat. No. 5,105,453. Without exception, radiation of one wavelength only is used in these examples, and consequently one obtains only one indication of the radiation intensity along each radiation path through the object to be measured. This indication may be used to determine the mean density of the object to be measured and changes thereof along the object but not to distinguish the presence of and the amount of different types of materials upon each measurement. In accordance with the examples given such information may be gained only from a large number of radiation paths that depart from different points (tomography).